The 6550 Experience
Moderators: VelvetGeorge, BUG
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
thank you Thorleif, so it's to protect the tubes that these can be lowered, ok.
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
Hi guys, so i read again the whole thread, in diagonal. i realise that i needed it.
i'll do a resumé of what i noted, from the west coast experiments or advices that differ a bit from one to another.
please correct or complete me if i'm wrong.
the bright cap :
- shakti went back and forth with the bright cap, testing 5000 pf, also 1500 pf, and finaly decided to remove it along the tests and experiments.
the NFB :
- Shakti has 47k
- CJH has 27k, but it's not a uperlead, it's more a single rectifier amp, with lower mains and preamp cap values
- Dave recommends to test between 47k and some lower values for some different saturation caracteristics.
i must say that i quite like this idea that i hear into cjh clips maybe.
mismatching :
- shakti recommends a 8R to 16R cab mismatch. Bill maybe too, i don't remember.
the bias résistors :
- Roe recommends to lower the 2 x 220k to 82k or even 68k.
- Dave didn't change it, and says the tube can get worn at some point.
- Bill didn't change it
- shakti didn't change it and it works fine so far.
the bias dropping resistor :
- shakti recommends to go from 27k to 10k.
the phase inverter cathode :
- Bill asks about 470 + 22k phase inverter cathode, but i didn't really understand that part.
the V2A cathode cap :
- shakti recommends to test with and without this 0,68 uf but he or someone else prefers with it in the end.
the 50/50 uf 500V mains and preamp filter :
- Dave did use some 100/100uf for the mains, and i'm not sure about the preamp cap value.
- everyone sticked to 50/50 , and cjh to its own values.
the 1k 5w screens résistors :
- big part of the mod, Dave recommends some 2k2 10 w
- almost everybody tried the 2k2 10w
the heaters :
- Roe, Bill, Shakti recommend to use two 0,1R résistors in parallel before the first tube, in case of use of the 220v tap, with 230 - 240 volts , to increase the voltage to about 540 volts. so the heaters are regulated from about 7v to 6,3 volts as it should be.
i'll do a resumé of what i noted, from the west coast experiments or advices that differ a bit from one to another.
please correct or complete me if i'm wrong.
the bright cap :
- shakti went back and forth with the bright cap, testing 5000 pf, also 1500 pf, and finaly decided to remove it along the tests and experiments.
the NFB :
- Shakti has 47k
- CJH has 27k, but it's not a uperlead, it's more a single rectifier amp, with lower mains and preamp cap values
- Dave recommends to test between 47k and some lower values for some different saturation caracteristics.
i must say that i quite like this idea that i hear into cjh clips maybe.
mismatching :
- shakti recommends a 8R to 16R cab mismatch. Bill maybe too, i don't remember.
the bias résistors :
- Roe recommends to lower the 2 x 220k to 82k or even 68k.
- Dave didn't change it, and says the tube can get worn at some point.
- Bill didn't change it
- shakti didn't change it and it works fine so far.
the bias dropping resistor :
- shakti recommends to go from 27k to 10k.
the phase inverter cathode :
- Bill asks about 470 + 22k phase inverter cathode, but i didn't really understand that part.
the V2A cathode cap :
- shakti recommends to test with and without this 0,68 uf but he or someone else prefers with it in the end.
the 50/50 uf 500V mains and preamp filter :
- Dave did use some 100/100uf for the mains, and i'm not sure about the preamp cap value.
- everyone sticked to 50/50 , and cjh to its own values.
the 1k 5w screens résistors :
- big part of the mod, Dave recommends some 2k2 10 w
- almost everybody tried the 2k2 10w
the heaters :
- Roe, Bill, Shakti recommend to use two 0,1R résistors in parallel before the first tube, in case of use of the 220v tap, with 230 - 240 volts , to increase the voltage to about 540 volts. so the heaters are regulated from about 7v to 6,3 volts as it should be.
Last edited by Xplorer on Sun Sep 20, 2015 9:10 am, edited 3 times in total.
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
.... Now my amp, as it is currently, all stock i think :
- the bright cap : 500 pf
- the tone stack : 500pf
- the bias résistors : 2 x 220k
- the bias droppping resistor : 27k
- the mains : 50/50 uf 500v
- the preamp cap : 50/50uf 500v
- the screens résistors : 1k 5w
- the nfb : 47k
here are some pictures. you can see some 12at7 and 100/100uf 400v caps from Dave, originaly planned to be used into a 45/100 as a base for the mod. they're smaller than the SL caps, i don't know if i'll use them for now.
also some 6550 tung sol, and résistors for the screens and for the heaters.



- the bright cap : 500 pf
- the tone stack : 500pf
- the bias résistors : 2 x 220k
- the bias droppping resistor : 27k
- the mains : 50/50 uf 500v
- the preamp cap : 50/50uf 500v
- the screens résistors : 1k 5w
- the nfb : 47k
here are some pictures. you can see some 12at7 and 100/100uf 400v caps from Dave, originaly planned to be used into a 45/100 as a base for the mod. they're smaller than the SL caps, i don't know if i'll use them for now.
also some 6550 tung sol, and résistors for the screens and for the heaters.



- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
Just finished the mod, will take more care of the Bias and report later.
i didn't really measure anything, not even the B+, and not even the heaters without the tubes in.
i just set the mv at 25 mv , but it could be all wrong, i was a bit impatient.
got to re learn how to bias, but with 6550 this time.
6550 are 25 watts like the el34 ?
i didn't really measure anything, not even the B+, and not even the heaters without the tubes in.
i just set the mv at 25 mv , but it could be all wrong, i was a bit impatient.
got to re learn how to bias, but with 6550 this time.
6550 are 25 watts like the el34 ?
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
i may need some little advice if possible please.
i can't reach 3,15 vac on the heaters.
well, i'm using two 0,3 ohm 7 watts in parallel to each other, ( measured to about 0,2 ohm ) connected between one tap of the PT, and the pin 7 of V7 .
on this side it goes down to 3,5 vac ... on the other side it's 3,6ac.
i'm starting to think that i'm completely wrong, and actualy each resistor should be connected between :
1 > 0,3 ohms 7 watts > the PT green tap and V7 pin 2
2 > 0,3 ohm 7 watts > the second green PT tap and V7 pin 7
i'm just wondering if 7 watts for each will be enough.
0,3 ohms might not even be enough , even if the recommended value was only 0,1 ohm ( the seller in the shop gave me these when i asked for some 0,1 ohms )
EDIT : now that each resistor is connected to one pin ( pins 2 and 7 of V7 ) , the voltage goes down to 3,4 Vac.
it makes more sense.
better than before, we'll go from there, i just hope that 7 watts each is enough. later i'll buy some higher values for the résistors.
i can't reach 3,15 vac on the heaters.
well, i'm using two 0,3 ohm 7 watts in parallel to each other, ( measured to about 0,2 ohm ) connected between one tap of the PT, and the pin 7 of V7 .
on this side it goes down to 3,5 vac ... on the other side it's 3,6ac.
i'm starting to think that i'm completely wrong, and actualy each resistor should be connected between :
1 > 0,3 ohms 7 watts > the PT green tap and V7 pin 2
2 > 0,3 ohm 7 watts > the second green PT tap and V7 pin 7
i'm just wondering if 7 watts for each will be enough.
0,3 ohms might not even be enough , even if the recommended value was only 0,1 ohm ( the seller in the shop gave me these when i asked for some 0,1 ohms )
EDIT : now that each resistor is connected to one pin ( pins 2 and 7 of V7 ) , the voltage goes down to 3,4 Vac.
it makes more sense.
better than before, we'll go from there, i just hope that 7 watts each is enough. later i'll buy some higher values for the résistors.
-
- Senior Member
- Posts: 210
- Joined: Sat Jan 10, 2015 9:43 am
- Just the numbers in order: 13492
- Contact:
Re: The 6550 Experience
Well am not familiar with the heater resistor mod.. but if you put them across the heater winding, its not very good.. your probably going to burn the heater wiring over time because of excessive current..or i missunderstood and you put them in serie. It was not clear to me.
P.S. i hate V2 cathode bypass cap.
P.S. i hate V2 cathode bypass cap.
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
Cooool
i definitely hear some interesting Hendrix accents in there, from new rising sun to of course bog, etc ...
but i should re write a voltage report because it's strange, now the heaters show 2,2 volts AC ...
( maybe since i mismatched the amp and cab : 8 ohms into a 16 ohms cab ( attenuator ) . i don't know.
the B+ that shows up when i put the standbye switch on shows 518 volts at max, when the bias is at 29 mV .
more ( about 530 volts ) when it's arround 14 mV .
the amp reacts so differently compared to when it had el34, it's really a lot closer to a 45/100, but without the acdc kind of saturation when crancked , it's different.
the blue fuzz doesn't seem to work as well as when i use it with the 45/100 though. but i still use it to thicken up the tone a bit, and i roll down the input bias on the fuzz, it helps cleaning too much darkened tone.
now the real tests should be made in a good environment, at the right volume, on a good cab.
so this should be it, i just think that there's some room for improvements on what i did, but even with the wrong transformer compared to the original, even with some 6550 reissue, it's getting there ...

i definitely hear some interesting Hendrix accents in there, from new rising sun to of course bog, etc ...
but i should re write a voltage report because it's strange, now the heaters show 2,2 volts AC ...
( maybe since i mismatched the amp and cab : 8 ohms into a 16 ohms cab ( attenuator ) . i don't know.
the B+ that shows up when i put the standbye switch on shows 518 volts at max, when the bias is at 29 mV .
more ( about 530 volts ) when it's arround 14 mV .
the amp reacts so differently compared to when it had el34, it's really a lot closer to a 45/100, but without the acdc kind of saturation when crancked , it's different.
the blue fuzz doesn't seem to work as well as when i use it with the 45/100 though. but i still use it to thicken up the tone a bit, and i roll down the input bias on the fuzz, it helps cleaning too much darkened tone.
now the real tests should be made in a good environment, at the right volume, on a good cab.
so this should be it, i just think that there's some room for improvements on what i did, but even with the wrong transformer compared to the original, even with some 6550 reissue, it's getting there ...
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
accross the heaters winding ? like a short ? no , i maybe misunderstand what you meant, but there's one 0.3 ohm resistor going between one green tap of the transformer to pin 7 of V7 , and another 0,3 ohm resistor going between the other green tap of the transformer and pin 2 of v7 , is it wrong ? shit, could it be the reason why the heaters now show 2,2 volts AC ?Tek465b wrote:Well am not familiar with the heater resistor mod.. but if you put them across the heater winding, its not very good.. your probably going to burn the heater wiring over time because of excessive current..or i missunderstood and you put them in serie. It was not clear to me.
P.S. i hate V2 cathode bypass cap.
i did like this because the first solution couldn't reduce the voltage enough, and it reduced it only on one side ( only on the pin 7 side, not the pin 2 side ) .
the V2 cathode cap ? i should try without it then, i don't know what change it produces.
EDIT : alright, it's just the 7 watts résistors i've used, they drifted already ... 7 watts wasn't enough maybe, i don't know.
i just had to measure before the resistor, and it still shows 3,6 volts there.
EDIT : on a real cab, actualy the blue fuzz works nicely when the volume is at max, and the fuzz gain from zero to 3 for example , it thickens the tone. i need it or the tone is quite too bright, so i still continue to think that BOG cleans were with a fuzz anyway, ( it brings back to the always ON fuzz with the wild switch .. ) . although ... the second part of "burning desire" in the bog show may contradict what i say, unless it's just strat vol rolled down a bit.
it's just like Dave said ( volume way up, fuzz gain quite low ) and spot on.
definitely an amp to explore ! i'm so glad to have this privilege to see this "absurd" old dream of mine becoming true, observing with my hands and ears this sound.
i'll really need two superleads, because i also really like when it has el34 in it.
so this is a first test for me with the west coast mod , i certainly didn't finish the tests.
later i'll need to have this mod in a new old stock Superlead clone , with a good PT like the dynaco, for more voltage and other qualitys. i feel that the effect would show up more at higher voltage, no ?
for now it's 530 volts.
this one will go back to EL34 then.
Thanks so much Dave ! i can hear it now !
will post clips and pictures later.
it makes me laugh now when i think about the Superlead EL34 VS JTM 45/100

it's neither and both at the same time.
Last edited by Xplorer on Sun Sep 20, 2015 10:37 pm, edited 4 times in total.
-
- Senior Member
- Posts: 713
- Joined: Wed Oct 29, 2014 9:36 pm
- Just the numbers in order: 13492
Re: The 6550 Experience
Apparently your PT has a center tap filament winding. This means there will be approximately half the voltage on either side of ground, if the center tap is grounded. This voltage does not have to be balanced; you can have 4 volts on one side and 2.3 volts on the other, what matters is that you get at least 6 VAC across the filament of the tube. It sounds like your filament voltage is way too low with a resistor on either half of the winding i.e.4.4 VAC. You should parallel the resistors and put them back on one of the filament leads so you get at least 6VAC across pins 2 and 7 of the 6550s, otherwise it won't respond correctly and you won't get the BOG sounds you are looking for.
-
- Senior Member
- Posts: 713
- Joined: Wed Oct 29, 2014 9:36 pm
- Just the numbers in order: 13492
Re: The 6550 Experience
If your filament voltage won't go low enough, just use (1).3 ohm resistor in series with one of the green heater wires. It may get hot, in which case you will need a 20 watt resistor.
-
- Senior Member
- Posts: 210
- Joined: Sat Jan 10, 2015 9:43 am
- Just the numbers in order: 13492
- Contact:
Re: The 6550 Experience
Remove the V2 cathode cap, it will get rid of that nasty thick tone.. and it will sound ALOT better.
Low heater voltage is not good for the tubes. premature wear.
edit: ok the green wire you refer to is the heater supply wire(not the chassis ground/heater ct).. forget anything i said before about heater.
Low heater voltage is not good for the tubes. premature wear.
edit: ok the green wire you refer to is the heater supply wire(not the chassis ground/heater ct).. forget anything i said before about heater.
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
thanks, actualy i do get the bog tone this way anyway, + a lot of other Hendrix tones, quite close to this one for exampledaveweyer wrote:Apparently your PT has a center tap filament winding. This means there will be approximately half the voltage on either side of ground, if the center tap is grounded. This voltage does not have to be balanced; you can have 4 volts on one side and 2.3 volts on the other, what matters is that you get at least 6 VAC across the filament of the tube. It sounds like your filament voltage is way too low with a resistor on either half of the winding i.e.4.4 VAC. You should parallel the resistors and put them back on one of the filament leads so you get at least 6VAC across pins 2 and 7 of the 6550s, otherwise it won't respond correctly and you won't get the BOG sounds you are looking for.
( i invite you guys to hear it entirely ... this version is really interesting. the drum and the basse look so modern for 1969, it's very special )
https://www.youtube.com/watch?v=UavFmtI32Ac
anyway, these resistors drifted. i previously had 7,2 volts AC supplied to the heaters, so too much. and now it's become 4,4 volts. i'll try to find some nicer resistors, 10 watts this time, and the right value so i get 3,15 volts and 3,15 volts as it should be.
this amp sounds fabulous with the univibe ( intensity zero, or higher ) and the blue fuzz. the g12h30 75hz pre rola sounds fabulous with it.
i get some results a little bit different from the clips that have been posted previously, but it's very close.
what's missing, appart from the right power transformer - and Dave .... did you recommend another output transformer also ? - is the 100/100 uf caps, ( also on the preamp ? ) , maybe play with the NFB, do some tests, why not the V2 cap ( but i don't get nasty thick tones anyway. ) , and play it loud !
Last edited by Xplorer on Sun Sep 20, 2015 10:56 pm, edited 1 time in total.
- Xplorer
- Senior Member
- Posts: 2480
- Joined: Sun Apr 19, 2009 5:27 pm
- Just the numbers in order: 7
Re: The 6550 Experience
thanks anyway tek, and i might try the v2 cap out , once i solve and secure the heaters's voltage issue.Tek465b wrote:Remove the V2 cathode cap, it will get rid of that nasty thick tone.. and it will sound ALOT better.
Low heater voltage is not good for the tubes. premature wear.
edit: ok the green wire you refer to is the heater supply wire(not the chassis ground/heater ct).. forget anything i said before about heater.
-
- Senior Member
- Posts: 210
- Joined: Sat Jan 10, 2015 9:43 am
- Just the numbers in order: 13492
- Contact:
Re: The 6550 Experience
Sorry i meant thin tone, not thick tone.. i was asleep when i typed that yesterday nightXplorer wrote:thanks anyway tek, and i might try the v2 cap out , once i solve and secure the heaters's voltage issue.Tek465b wrote:Remove the V2 cathode cap, it will get rid of that nasty thick tone.. and it will sound ALOT better.
Low heater voltage is not good for the tubes. premature wear.
edit: ok the green wire you refer to is the heater supply wire(not the chassis ground/heater ct).. forget anything i said before about heater.

In your previous post you said it was a bit too bright. thats why i said to remove it.
In my amp, from my experience that V2 cathode cap make for compressed thin highs and too bright sound.
-
- Senior Member
- Posts: 713
- Joined: Wed Oct 29, 2014 9:36 pm
- Just the numbers in order: 13492
Re: The 6550 Experience
Lots of folks don't like the V2 bypass cap because it makes the channel too bright. You can either get rid of it, or alternatively use a .02 coupling cap instead of the .002, have had some luck with using a .01 in that spot, even a .005.
This method gives a little bit more overall gain.
This method gives a little bit more overall gain.